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Limited evidence from clinical samples appears to support an inverse relationship between current substance use disorders and obesity. Persons who seek bariatric surgery for obesity report relatively high rates of lifetime substance use disorders but very low rates of current substance use disorders. This suggests that abstaining from addictive drugs could contribute to obesity.20 Other studies find inverse relationships between BMI and past-year alcohol and marijuana use among women seeking weight loss treatment.21,22 Among individuals with bipolar disorder, there is an inverse relationship between overweight/obesity and substance use disorders.23

Compulsive overeating and addictions to alcohol and other drugs appear to share common psychological and physiological underpinnings. Intake of food or drugs is reinforcing and activates reward circuits in the brain, causing the release of dopamine.24 The reward response to palatable food encourages eating, thus promoting survival; however, substances of abuse activate the system as well.25 Acute substance use leads to increased concentrations of dopamine in the brain. With chronic excessive use, however, the number of dopamine receptors declines, which leads to an eventual reduction in dopamine activity.24 A similar process may occur with overeating.

Wang and colleagues26 used positron emission tomography (PET) to compare concentrations of dopamine D2 receptors in the brains of extremely obese and normal-weight individuals. Obese individuals had significantly fewer D2 receptors than normalweight individuals. Among the obese individuals, D2 receptor concentration declined as body weight increased.26 These findings suggest that overeating, like substance use, may initially stimulate dopamine activity but eventually leads to down-regulation of dopamine receptors and a reduction in dopamine activity.

However, some authors have postulated a preexisting "reward deficiency syndrome" that predisposes one to compulsively engage in rewarding activities, including substance abuse, overeating, gambling, and sexual addiction behavior.27 Variations in genes that govern expression of dopamine D2 receptors lead to individual differences in receptor density. Individuals with fewer D2 receptors are less sensitive to subtle rewards and more prone to anxiety, anger, and dysphoria, which leaves them vulnerable to overindulging in rewarding behaviors in an effort to alleviate negative emotions.27

Overeating may thus be one of several behaviors used to compensate for the blunted reward effects of a hypoactive dopamine system. If that is the case, individuals examined by Wang and colleagues26 may have become obese because of a shortage of dopamine receptors rather than a loss of receptors because of overeating. Although there is evidence for a common pathway to obesity or drug addiction, the lack of consistent associations between obesity and substance use disorders in epidemiological samples suggests that any relationship between the two is complex.

Associations with other psychiatric disorders

Obesity is associated with several personality disorders in the NESARC sample.5 Antisocial, avoidant, obsessive-compulsive, paranoid, and schizoid personality disorders are all more prevalent among the obese and extremely obese than in normal-weight persons (OR, 1.31 - 2.55). In addition, extreme obesity is associated with a greater likelihood of dependent personality disorder (OR, 3.04). Antisocial personality disorder (ASPD) is significantly associated with BMI among women.28

Obesity has been associated with elevated rates of attention-deficit/hyperactivity disorder (ADHD).29,30 Children with ADHD and other disruptive behavior disorders are heav-ier than their peers without behav-ioral disorders and are likely to remain overweight into adulthood.31 Impulsivity associated with ADHD may thus contribute to overeating and obesity.

Schizophrenia is not associated with obesity after controlling for other variables.32 However, treatment with antipsychotic medications—particularly olanzapine and clozapine—can lead to substantial weight gain in some patients.32,33

Overall, the accumulated data on obesity and psychiatric disorders indicate that obese persons may face a variety of mental health problems in addition to the physical health problems that often accompany obesity.